This invention relates to a relief patterned sheet having optical properties useful in a wide variety of applications, as for example in the production of relatively thin, flexible fresnel lenses, in decorative sheets or laminates having various decorative patterned optical effects, in projection screens, and in lenticular screen sheets of the type which are useful for forming animated display signs or producing three dimensional optical effects.
This type of sheet material is characterized by having a predetermined relief pattern formed in the surface thereof. The particular relief pattern depends upon the intended use of the sheet material. For example, the relief pattern may comprise a series of fine closely spaced lenslike formations designed so as to reflect or refract light in a particular manner for bringing about the desired optical effect.
In order to obtain the desired optical characteristics in this type of sheet material, it is highly important that the relief patterned surface be of high quality and fine definition. Heretofore, in order to achieve the high level of quality and definition needed for this type of sheet material, stamping or embossing by a platen press was the best available manufacturing method. This method involves placing a thermoplastic material in a platen press having an engraved surface corresponding to the desired relief pattern. The platen press is closed and subjected to a heating cycle to heat and soften the thermoplastic material and cause it to conform to the engraved surface of the platen press. The platen press then undergoes a cooling cycle to cool and harden the thermoplastic material so that it will retain its shape and can be removed from the platen press.
Because of the required heating and cooling cycles, the platen press method is necessarily slow and relatively expensive. Additionally, the size of the sheet material is limited by the size of the platen press, and this method is thus not suited for producing a continuous length product.
While other processes are known for producing a relief patterned sheet in continuous length form, the quality and definition of the relief pattern is generally inferior to that obtainable by the platen press method. Typically, such continuous methods utilize an engraved roll and the relief pattern is formed by the roll by embossing the thermoplastic material while in a softened or molten state. Because of the elastic nature of the thermoplastic material and the internal stresses imparted by the embossing roll, the embossed sheet produced by this method has a tendency to try to return to its original flattened and unstressed configuration. Thus, the particular lenslike or other configurations which are formed in the sheet change in shape from their originally desired shape, losing some of their desired optical qualities. Additionally, sheets produced by this method are highly susceptible to streaking during the manufacturing operation as a result of condensation on the sheet during cooling. Also, the sheet is susceptible to dimensional distortion by the embossing machinery unless the roll speeds and web tensions are accurately maintained.
The quality of the relief patterned sheet materials produced by the above-described methods is also limited by the properties of the thermoplastic polymers from which such sheets are formed. For example, these thermoplastic polymers are subject to a number of potentially damaging influences, including staining, solvents damage, abrasion damage, distortion or change in dimension from heat or moisture, and migration of the plasticizers used therein, all of which can undesirably affect the optical qualities of the sheet.